A PM indicated some clarification of the graphic I posted above would be helpful for readers. I'm not sure if this will help or confuse more.
MY OPINION: I see one value of these nerdy TV-TVd-FL exercises, as above and explained (?) below, as helping me/us understand when there is need to look at lengthening Inspiratory times, unusually large and persistent I/E ratios, or better than I/E ratios, higher duty cycle ratios (Ti/(Ti+Te). Look at those indicators when sleep is unrestful with a low AHI. Look at them when the zoomed FL have high severity values or are of long duration; those FL flags may arise more from wave shape or breath timing than from drops in tidal volume. Shape and timing are important, but far overshadowed by TV need (as that ventilator crisis in ICUs and the EMTs' ABC rule-Airways first rule confirm).
Getting sufficiently regular tidal volume to support health and restful sleep is our goal. As others have pointed out, lengthening inspiratory time relative to expiratory time indicates more work is being done to get needed TV and there can be some or all the more usual negative consequences of apnea, maybe worse because sleep medicine often overlooks insidious flow limitations.
Redmed devices show your whole night's
I/E ratio 1/1.8 = 0.56, for example. Better than that, with OSCAR you can put the Inspiratory time curve above the Expiratory time curve and look at FL where curves diverge widest. Note the higher ratios, and how persistent they are and how they vary night to night. This is one of the ways to help decide if a certain food, Rx or activity is helping or hurting your sleep.
ONWARD: The graphic below was on hand from my own sleep file, easy to grab. It had been stripped down to its barest essentials to show in a close-up concepts presented--squeezed together--in the zoomed-out graphic above. Comment here is about the lower graphic and how it explains the graphic above.
It was assumed in my stock graphic below that normal tidal volumes (TV) vary around, say, 0.5 L. This means that a single normal/ideal area between the green axis and the inspiratory curve would, when calculated, amount to a 0.5 L TV. Green vertical bar lengths indicate actual measured TV of the wave to the left as determined from data on a Resmed data card file. Below, we see some flow limitation effects, "fL", in tips of the waves but no FL flag. The green colored bar lengths correspond to the lighter green traces in the graphic above where the light green traces also rise above the flow rate traces.
Breathing in the upper graph showed TV varying up and down relative to 0.4 L. All red descending bars represent tidal volume (TV) drops, (TVd), below 0.5 L below, and below 0.4 L above. The leftmost inspiratory wave below has a TVd = 0.5L - 0.4L = 0.1L as one can see from differences in values at left. The upper graph, a different layout scheme, would (correctly) show a TVd of -0.1L. Nevertheless, the method is the same in the graphics above and below; main point: Tidal volume drop (TVd) is equal to the difference between the assumed baseline TV (="TVb", say) of 0.5 L, and the measured/calculated tidal volume TV. (In the graphic above breaths with TV larger than 0.4 "pushed" the red bars above the zero-TVd axis--that red "fuzz" or "negative TVd drops" that project above the axis). One note about TV, we tend to breathe deeper or longer or faster (faster and shallower being less efficient) to maintain a fairly steady TV if we are physically able to, apneas aside.
The graphic above shows some TVd-FL flag "agreement" (a) where red bars are overlaid by yellow to make rust color ("disagreeing" yellow color may extend beyond rust color) and (b) where the red bars are smallest without yellow or rust colors. Accordingly, the rust color blend and small red bars indicate some kind of concordance of FL flags and TVd bars. The yellow color extending beyond the rust color suggests, but does not mean FL severity, as scaled, is more serious than is indicated by TVd red bars. A Resmed FL flag,
best I can determine, can simply signal the device algorithm has detected a wave shape or wave timing value it is "wary" of in this sleep context but there is no significant TVd to report.
A second graphic illustrates how TV values were determined by using Excel to add up all strip areas making up each TV. Each single-strip volume = .04 (second) X flow rate (L/sec) as both are logged in the Resmed BRP data file.
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